U.S. Pat. No. 4,975,354 issued Dec. 4, 1990, entitled "Photographic Element Comprising An Ethyleneoxy-Substituted Amino Compound And Process Adapted To Provide High Contrast Development", by Harold I. Machonkin and Donald L. Kerr, describes silver halide photographic elements having incorporated therein a hydrazine compound which functions as a nucleator and an amino compound which functions as an incorporated booster. Such elements provide a highly desirable combination of high photographic speed, very high contrast and excellent dot quality, which renders them very useful in the field of graphic arts. Moreover, since they incorporate the booster in the photographic element, rather than using a developing solution containing a booster, they have the further advantage that they are processable in conventional, low cost, rapid-access developers.
While the high-contrast photographic elements of U.S. Pat. No. 4,975,354 represent a major advance in the art, there is a continuing need to improve the properties of these photographic elements, for example, to provide increased photographic speed and even higher contrast. Moreover, enhanced developability of these high-contrast elements, which would enable the use of very short development times, would also be highly beneficial in the field of graphic arts.
It is a well known expedient to increase photographic speed by the use of chemical sensitizing agents, and a very wide variety of different compounds are known to be useful as chemical sensitizers (see, for example, Research Disclosure, Issue No. 308, Item 308119, Paragraph III, December, 1989). However, the use of chemical sensitizing agents can adversely affect other properties of silver halide emulsions which contain a hydrazine compound that functions as a nucleator and an amino compound that functions as an incorporated booster, for example, it can adversely affect contrast or result in an increase in fog.
Sodium thiosulfate is commonly used as a chemical sensitizing agent for photographic silver halide emulsions. Thiourea is also a well known chemical sensitizing agent as described, for example, in U.S. Pat. No. 1,623,499 However, as shown herein, sodium thiosulfate and thiourea are not effective chemical sensitizers for emulsions that contain a hydrazine compound that functions as a nucleator and an amino compound that functions as an incorporated booster.
Use of sulfur sensitizers, including thioureas, as compounds which are capable of selectively sensitizing the (100) plane of silver halide grains is described in European Patent Application No. 0 302 528, published Feb. 8, 1989. However, the emulsion systems described in this patent application do not contain a hydrazine compound that functions as a nucleator nor an amino compound that functions as an incorporated booster.
Certain 1,1,3,3-tetra-substituted middle chalcogen urea compounds that are highly effective chemical sensitizers are described in U.S. Pat. No. 4,810,626. Use of these compounds as sensitizers in high-chloride emulsions that contain a hydrazine compound that functions as a nucleator and an amino compound that functions as an incorporated booster is described in the aforesaid copending commonly assigned U.S. patent application Ser. No. 735,979, filed Jul. 25, 1991.
It is toward the objective of providing an improved high-contrast photographic element--containing both a hydrazine compound that functions as a nucleator and an amino compound that functions as an incorporated booster--that has enhanced developability and that exhibits increased speed and increased toe contrast, without a concurrent increase in fog, that the present invention is directed.